More than 350 distinct arthropod-borne viruses (arboviruses) are currently characterized of which between 20 and 25 are known to cause significant disease of man in nature, either in terms of numbers involved, severity of illness or both. The majority belong in one of the following taxons: togaviridae, bunyaviridae, rhabdoviridae and reoviridae (orbivirus). Among the diseases caused by arboviruses are several types of encephalitides, hemorrhagic fevers and yellow fever. Vaccination has effectively ended the menace of urban yellow fever; however, there still are numerous problems that have not been resolved concerning ecology, prevention, diagnosis and basic properties of the arboviruses. The central, unifying theme of the submitted projects is a better understanding of the properties and activity of the arboviruses in order to apply this knowledge to the control and prevention of infection of man and domestic animals. The projects involve studies on three separate but inter-related areas, ecology, epidemiology and immunity; the projects have been chosen for their intrinsic importance as well as for their value as general models. Project #1, on Crimean-Congo hemorrhagic methods, geographical distribution of antigeneic varieties and immune reactivity of subviral units. Project #2 is immunologically directed and proposes to investigate the effect on the course of an infection of immune serum given after exposure to the virus. Projects #3 and #4 are ecologically oriented in attempting to clarify and understand the mechanisms of vector susceptibility to denuge and yellow fever viruses, particularly with Aedes aegypti, #3; and to rabies-related arboviruses mainly utilizing virus propagated in A. albopictus cell cultures to infect mosquitoes, #4. Finally, project #5, basically oriented, investigates mechanism accounting for neurovirulence, particularly as these mechanisms relate to the cell membrane.